Pavement marking, reflective elements, and methods of making microspheres

ABSTRACT

Presently described are retroreflective articles, such as pavement markings, that comprise transparent microspheres partially embedded in a (e.g., polymeric) binder. Also described are (e.g., glass-ceramic) microspheres, methods of making microspheres, as well as compositions of glass materials and compositions of glass-ceramic materials. The microspheres generally comprise lanthanide series oxide(s), titanium oxide (TiO 2 ), and optionally zirconium oxide (ZrO 2 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/273,513, filed Nov. 14, 2005.

FIELD

The invention relates to articles such as retroreflective pavementmarkings and other retroreflective articles as well as retroreflectiveelements comprising microspheres, methods of making microspheres,microspheres, and compositions of glass as well as glass-ceramicmaterials.

BACKGROUND

Transparent glass and glass-ceramic microspheres (i.e., beads) are usedas optical elements for retroreflective signage, apparel, and pavementmarkings. Such microspheres can be produced, for example, by meltingmethods. Such melting methods may include melting a raw material mixturein the form of particulate material. The melted particles can bequenched, in air or water for example, to give solid beads. Optionally,quenched particles can be crushed to form particles of a smaller desiredsize for the final beads. The crushed particles can be passed through aflame having a temperature sufficient to melt and spheroidize them. Formany raw material compositions this is a temperature of about 1500° C.to about 3000° C. Alternatively, the melted raw material composition canbe poured continuously into a jet of high velocity air. Molten dropletsare formed as the jet impinges on the liquid stream. The velocity of theair and the viscosity of the melt are adjusted to control the size ofthe droplets. The molten droplets are rapidly quenched, in air or waterfor example, to give solid beads. Beads formed by such melting methodsare normally composed of a vitreous material that is essentiallycompletely amorphous (i.e., noncrystalline), and hence, the beads areoften referred to as “vitreous,” “amorphous,” or simply “glass” beads ormicrospheres.

Pavement markings including microspheres prepared from compositions thatcomprise lanthanum oxide and titanium oxide are described for example inU.S. Pat. No. 3,946,130 (Tung) and WO 96/33139.

SUMMARY

In one embodiment, a method of marking a pavement surface is describedcomprising providing a pavement surface and applying a pavement markingon the pavement surface. The pavement marking comprises transparentmicrospheres at least partially embedded in a binder wherein themicropheres comprise at least 40 wt-% TiO₂, and at least 10 wt-% La₂O₃;and the microspheres have an index of refraction of at least 2.10.

In another embodiment, the pavement marking comprises transparentmicrospheres at least partially embedded in a binder wherein themicropheres comprise at least 50 mol-% TiO₂, at least 5 mol-% of one ormore metal oxides selected from oxides of the lanthanide serieselements; and the microspheres have an index of refraction of at least2.10.

In another embodiment, the pavement marking comprises transparentmicropheres at least partially embedded in a binder. At least a portionof the micropheres comprise at least 50 mol-% TiO₂, at least 5 mol-%Y₂O₃, and optionally at least 5 mol-% zirconia, hafnia, thoria, andmixtures thereof.

In another embodiment, a retroreflective element is described. Theretroreflective elements comprise the microspheres described hereinpartially embedded in an organic or inorganic core.

In other embodiments, methods of producing microspheres are described.The method comprises providing materials of the starting compositionsdescribed herein, melting the starting materials with a flame at a flametemperature of less than 2700° C. to form molten droplets, cooling themolten droplets to form quenched fused microspheres, and optionallyheating the quenched fused microspheres.

In other embodiments, retroreflective articles, (e.g., glass-ceramic)microspheres, as well as glass compositions and glass-ceramiccompositions are described.

For each embodiment that includes microspheres, the microspheres mayhave an index of refraction of at least 2.20, at least 2.30 or at least2.40. The micropheres may comprise a glass-ceramic structure. Thecomposition may comprise 45 wt-% to 70 wt-% TiO₂; 20 wt-% to 55 wt-% ofLa₂O₃; or one or more metal oxides selected from oxides of thelanthanide series elements; and 5 to 25 wt-% ZrO₂, HfO₂, ThO₂, andmixtures thereof. The composition may comprises at least 25 wt-% La₂O₃or one or more metal oxides selected from oxides of the lanthanideseries elements. The composition may comprise at least 50 wt-% or atleast 60 wt-% TiO₂. The composition may comprise 5 wt-% to 10 wt-% ZrO₂,HfO₂, ThO₂, and mixtures thereof.

One exemplary composition comprises 60 wt-% to 65 wt-% TiO₂, 25 wt-% to35 wt-% of La₂O₃ or one or more metal oxides selected from oxides of thelanthanide series elements; and 5 to 10 wt-% ZrO₂, HfO₂, ThO₂ andmixtures thereof.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an illustrative retroreflectiveelement in accordance with the invention.

FIG. 2 is a perspective view of an illustrative pavement marking.

FIG. 3 is a cross-sectional view of an illustrative pavement markingtape of the invention.

FIG. 4 is an X-ray diffraction plot for exemplary glass beads andexemplary glass-ceramic beads of the invention.

FIG. 5 is an optical photomicrograph of the surface of a retroreflectivearticle demonstrating beads partially embedded in a binder.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Presently described are retroreflective articles, such as pavementmarkings, that comprise transparent microspheres partially embedded in a(e.g., polymeric) binder. Also described are (e.g., glass-ceramic)microspheres, methods of making microspheres, as well as compositions ofglass materials and compositions of glass-ceramic materials. Themicrospheres generally comprise lanthanide series oxide(s) (e.g.,La₂O₃), titanium oxide (TiO₂), and optionally zirconium oxide (ZrO₂).This base composition will be referred to herein as “LTZ”. Beads thatinclude the LTZ base composition will be referred to as “LTZ beads” or“LTZ microspheres.”

The terms “beads” and “microspheres” are used interchangeably and referto particles that are substantially spherical.

The term “solid” refers to beads that are not hollow, i.e., free ofsubstantial cavities or voids. For use as lens elements, the beads arepreferably spherical and preferably solid (i.e., non-porous). Solidbeads are typically more durable than hollow beads. Solid beads can alsofocus light more effectively than hollow beads, leading to higherretroreflectivity.

The microspheres described herein are preferably transparent. The term“transparent” means that the beads when viewed under an opticalmicroscope (e.g., at 100×) have the property of transmitting rays ofvisible light so that bodies beneath the beads, such as bodies of thesame nature as the beads, can be clearly seen through the beads whenboth are immersed in oil of approximately the same refractive index asthe beads. Although the oil should have an index of refractionapproximating that of the beads, it should not be so close that thebeads seem to disappear (as they would in the case of a perfect indexmatch). The outline, periphery, or edges of bodies beneath the beads areclearly discernible.

The recitation of numerical ranges by endpoint includes all numberssubsumed within the range (e.g., the range 1 to 10 includes, forexample, 1, 1.5, 3.33, and 10).

Beads of the present invention are particularly useful as lens elementsin retroreflective articles. Transparent beads according to the presentinvention typically have an index of refraction of at least 2.10. Forretroreflective applications in water or a wet environment, the beadspreferably have a high index of refraction of at least 2.20, of at least2.25, of at least 2.30, of at least 2.35, and preferably of at leastabout 2.40.

Although high index of refraction glass beads have been demonstrated inthe past for compositions comprising large amounts of titania as well(U.S. Pat. No. 3,493,403), such beads were prepared with a plasma torchthat provided a fusion temperature reported to be in excess of 5000° F.(2760° C.). Also, such beads were prepared from particles less than 90micrometers in size. Finally, such transparent fused beads were formedby rapidly quenching in water. Particles less than 90 micrometers insize heat rapidly in a plasma torch, and also quench at higher ratesthan larger particles, due to increasing heat transfer with decrease ina particle size. Thus, compositions that can be heated and quenched intoa transparent bead less than 90 micrometers in diameter, using a plasmatorch and water quenching, are often not suitable for preparation oftransparent beads with larger sizes, using lower temperature fusionequipment and air-quenching. In many applications, it is desired toprepare quenched fused beads with size larger than 90 micrometers. Forpractical and low-cost manufacturing, it is desirable to usecompositions that take advantage of equipment that provides a fusiontemperature less than 2760° C. (e.g., less than 2700° C., less than2600° C., less than 2500° C., less than 2400° C., less than 2300° C.,less than 2200° C., less than 2100° C., less than 2000° C.). Forexample, it is particularly advantageous if a solid transparent bead canbe formed using a flame produced by the combustion of natural gas withair, which is characterized by an adiabatic flame temperature ofapproximately 1980° C. It is also desirable to use equipment thatprovides only air-quenching. Thus, the present invention providescompositions with exceptional melting and glass-forming properties,making them useful for forming beads with size ranging above 90micrometers (e.g., 100 micrometers, 150 micrometers, 200 micrometers),using combustion flame fusion processes and air-quenching.

In addition to the advantages of being conveniently melted and quenchedto form transparent beads and having high index of refraction, beads ofthe current invention, when not doped intentionally to develop color,also exhibit higher whiteness, making them especially useful for whiteretroreflective sheeting and white retroreflective pavement markings. Byhigher whiteness, what is meant is that the beads appear less coloredthan past beads of such high index of refraction, for example beadscomprising mainly alkaline earth oxides, titania and zirconia.

Articles of the invention share the common feature of comprising the LTZbeads described herein and/or a reflective element comprising such beadsat least partially embedded in a core. At least a portion of the LTZbeads and/or reflective elements are exposed on the viewing surface ofthe article (e.g., pavement marking). The microspheres and/or reflectiveelements are preferably embedded in the core at a depth ranging fromabout 30% to about 60% of their diameters.

The pavement markings of the invention comprise a binder. In someaspects, the binder affixes the microspheres or the elements comprisingmicrospheres to a pavement surface. Pavement surfaces are typicallysubstantially solid and include a major portion of inorganic materials.Typically pavement surfaces include asphalt, concrete, and the like. Thebinder typically comprises a paint, a thermoplastic material, thermosetmaterial, or other curable material. Common binder materials includepolyacrylates, methacrylates, polyolefins, polyurethanes, polyepoxideresins, phenolic resins, and polyesters. For reflective pavement markingpaints the binder may comprise reflective pigment.

For reflective sheeting that is suitable for reflective signage,apparel, or other uses, the binder that affixes the beads is typicallytransparent. Transparent binders are applied to a reflective base or maybe applied to a release-coated support, from which after solidificationof the binder, the beaded film is stripped and may subsequently beapplied to a reflective base or be given a reflective coating orplating.

The reflective elements comprising microspheres and/or the microspheresof the invention are typically coated with one or more surfacetreatments that alter the pavement marking binder wetting propertiesand/or improve the adhesion of the reflective elements comprisingmicrospheres or the microspheres in the binder. The reflective elementsare preferably embedded in the pavement marking binder to about 20-40%,and more preferably to about 30% of their diameters such that thereflective elements are adequately exposed. Surface treatments thatcontrol wetting include various fluorochemical derivatives such ascommercially available from Du Pont, Wilmington, Del. under the tradedesignation “Krytox 157 FS”. Various silanes such as commerciallyavailable from OSI Specialties, Danbury, Conn. under the tradedesignation “Silquest A-1100” are suitable as adhesion promoters.

With reference to FIG. 1, retroreflective element 200 comprises LTZmicrospheres 117 alone or in combination with low index bead 116partially embedded in the surface of a core 202. The core is typicallysubstantially larger than the beads. For example the average corediameter may range from about 0.2 to about 10 millimeters.

The core may comprise an inorganic material. Glass-ceramics are alsouseful as a core material. The crystalline phase acts to scatter lightresulting in a semi-transparent or opaque appearance. Alternatively, thecore may comprise an organic material such as a thermoplastic or bondedresin core, i.e., a crosslinked cured resin such as an epoxy,polyurethanes, alkyds, acrylics, polyesters, phenolics and the like.Various epoxies, polyurethane, and polyesters are generally described inU.S. Pat. Nos. 3,254,563; 3,418,896; and 3,272,827. The core may be acomposite comprising an inorganic particle that is coated with anorganic material. In the latter case, the organic material serves as abinder to affix the beads to the outside surface of the core.

Although the retroreflective elements may be prepared from anon-diffusely reflecting bonded resin core in combination withspecularly reflecting microspheres (e.g., vapor coating the microsphereswith aluminum), this approach results in less durable retroreflectiveelements due to the use of metal which may be susceptible to chemicaldegradation. Less durable retroreflective elements would also result byincorporating metals (e.g., aluminum) into the core. In preferredembodiments, the retroreflective elements comprise at least onenon-metallic light scattering material dispersed within core. Theretroreflectance R_(A) of the reflective elements for an entrance angleof −4° and a 0.2° obervation angle is typically at least about 3(Cd/m²)/lux and preferably at least about 7 (cd/m²)/lux, when submergedin water.

Reflective elements may be made by known processes, such as described inU.S. Pat. Nos. 5,917,652; 5,774,265; and 2005/0158461-A1.

In some aspects, the beads and/or reflective elements are employed inliquid-applied marking (e.g., pavement) applications. With reference toFIG. 2, the beads 117 and/or reflective elements 200 are sequentially orconcurrently dropped onto a liquified binder 10 or compounded within aliquified binder that is provided on pavement surface 20.

In other aspects, beads and/or reflective elements are employed inretroreflective sheeting including exposed lens, encapsulated lens,embedded lens, or enclosed lens sheeting. Representativepavement-marking sheet material (tapes) are described in U.S. Pat. No.4,248,932 (Tung et al.); U.S. Pat. No. 4,988,555 (Hedblom); U.S. Pat.No. 5,227,221 (Hedblom); U.S. Pat. No. 5,777,791 (Hedblom); and U.S.Pat. No. 6,365,262 (Hedblom).

Pavement marking sheet material generally includes a backing, a layer ofbinder material, and a layer of beads partially embedded in the layer ofbinder material. The backing, which is typically of a thickness of lessthan about 3 millimeters, can be made from various materials, e.g.,polymeric films, metal foils, and fiber-based sheets. Suitable polymericmaterials include acrylonitrile-butadiene polymers, millablepolyurethanes, and neoprene rubber. The backing can also includeparticulate fillers or skid resistant particles. The binder material caninclude various materials, e.g., vinyl polymers, polyurethanes,epoxides, and polyesters, optionally with colorants such as inorganicpigments, including specular pigments. The pavement marking sheeting canalso include an adhesive, e.g., a pressure sensitive adhesive, a contactadhesive, or a hot melt adhesive, on the bottom of the backing sheet.

Pavement markings typically exhibit an initial R_(L) according to ASTM E1710-97 of at least 300 millicandelas/m²/lux, preferably at least 500millicandelas/m²/lux, more preferably at least 800 millicandelas/m²/lux,and even more preferably at least 1000 millicandelas/m²/lux.

Patterned retoreflective (e.g., pavement) markings advantageouslyprovide vertical surfaces, e.g., defined by protrusions, in which themicrospheres are partially embedded. Because the light source usuallystrikes a pavement marker at high entrance angles, the verticalsurfaces, containing embedded microspheres, provide for more effectiveretroreflection. Vertical surfaces also tend to keep the microspheresout of the water during rainy periods thereby improving retroreflectiveperformance.

For example, FIG. 3 shows patterned pavement marker 100 containing a(e.g., resilient) polymeric base sheet 102 and a plurality ofprotrusions 104. For illustrative purposes, only one protrusion 104 hasbeen covered with microspheres and antiskid particles. Base sheet 102has front surface 103 from which the protrusions extend, and backsurface 105. Base sheet 102 is typically about 1 millimeter (0.04 inch)thick, but may be of other dimension if desired. Optionally, maker 100may further comprise scrim 113 and/or adhesive layer 114 on back surface105. Protrusion 104 has top surface 106, side surfaces 108, and in anillustrative embodiment is about 2 millimeters (0.08 inch) high.Protrusions with other dimensions may be used if desired. As shown, sidesurfaces 108 meet top surface 106 at a rounded top portions 110. Sidesurfaces 108 preferably form an angle θ of about 70° at the intersectionof front surface 103 with lower portion 112 of side surfaces 108.Protrusion 104 is coated with pigment-containing binder layer 115.Embedded in binder layer 115 are a plurality of LTZ microspheres 117 anda plurality of a second microspheres 116 (e.g., having a lowerrefractive index than the LTZ microspheres). Optionally, antiskidparticles 118 may be embedded on binder layer 115.

Pavement marking sheetings can be made by a variety of known processes.A representative example of such a process includes coating onto abacking sheet a mixture of resin, pigment, and solvent, dropping beadsaccording to the present invention onto the wet surface of the backing,and curing the construction. A layer of adhesive can then be coated ontothe bottom of the backing sheet. U.S. Pat. No. 4,988,541 (Hedblom)discloses a preferred method of making patterned pavement markings andis incorporated herein by reference in its entirety. Optionally, a scrim(e.g., woven or nonwoven) and/or an adhesive layer can be attached tothe back side of the polymeric base sheet, if desired.

In some embodiments of the invention, two types of microspheres areemployed wherein one type are the LTZ beads described herein and thesecond type are “low index microspheres,” having for example arefractive index ranging from about 1.5 to about 2.0. In some aspects,one of the two types of microspheres will be larger. For instance, theoptional low index microspheres may range in diameter from 175 to 250micrometers in diameter while the LTZ microspheres are about 50 to 100micrometers in diameter. In such case, the smaller LTZ microspheres maybe disposed between the larger low index microspheres. As a result, theLTZ microspheres are protected against abrasion caused by repeatedtraffic wear. Alternatively, however, the LTZ microspheres can be chosento be larger than the optional low index microspheres. Typically, thelarger microspheres will cover more than about 50 percent of theretroreflective portion of the pavement marking surface area.

The optional low index microspheres are typically present in an amountof at least 25 weight percent, and preferably from about 35 to about 85weight percent of the total amount of microspheres used. The LTZmicrospheres are typically present from 15 to about 75 weight percent.These ranges are preferred because they provide a good balance betweendry and wet retroreflectivity and provide good abrasion resistance.

The microspheres are preferably placed selectively on the side and topsurfaces of the protrusions while leaving the valleys betweenprotrusions substantially clear so as to minimize the amount ofmicrospheres used, thereby minimizing the manufacturing cost. Themicrospheres may be placed on any of the side surfaces as well as thetop surface of the protrusions to achieve efficient retroreflection.

The binder layer of FIGS. 2 and 3 as well as the core of theretroreflective element depicted in FIG. 1 comprise a light transmissivematerial so that light entering the retroreflective article is notabsorbed but is instead retroreflected by way of scattering orreflection off of pigment particles in the light-transmissive material.Vinyls, acrylics, epoxies, and urethanes are examples of suitablemediums. Urethanes, such as are disclosed in U.S. Pat. No. 4,988,555(Hedblom) are preferred binder mediums at least for pavement markings.The binder layer preferably covers selected portions of the protrusionsso that the base sheet remains substantially free of the binder. Forease of coating, the medium will preferably be a liquid with a viscosityof less than 10,000 centipoise at coating temperatures.

The binder layer of FIGS. 2 and 3 as well as the core of FIG. 1typically comprise at least one pigment such as a diffusely reflectingor specularly reflecting pigment.

Specular pigment particles are generally thin and plate-like and arepart of the binder layer, the organic core (a core comprisingessentially only an organic binder material) of an element, or anorganic binder coating on an inorganic particle that together make up acomposite core of an element. Light striking the pigment particles isreflected at an angle equal but opposite to the angle at which it wasincident. Suitable examples of specular pigments for use in the presentinvention include pearlescent pigments, mica, and nacreous pigments.Typically, the amount of specular pigment present in the binder layer isless than 50 percent by weight. Preferably, the specular pigmentscomprise about 15 percent to 40 percent of the binder layer by weight,this range being the optimum amount of specular pigment needed forefficient retroreflection. Pearlescent pigment particles are oftenpreferred because of the trueness in color.

In lieu of or in addition to combining transparent beads with areflective (e.g., pigment containing) binder and/or element core, thebeads may comprise a reflective (e.g., metallic) coating. Preferrably,the metallic coating is absent from the portion of the outside surfaceof the bead that oriented to receive the light that is to beretroreflected, and present on the portion of the outside surface of thebead that is oriented opposite to the direction from which light that isto be retroreflected is incident. For example, in FIG. 1, a metalliccoating may be advantageously placed at the interface between bead 117and core 202. In FIG. 3, a reflective layer may be advantageously placedat the interface between the bead 117 and the binder 115 such as shownin U.S. Pat. No. 6,365,262. Metallic coatings may be placed on beads byphysical vapor deposition means, such as evaporation or sputtering. Fullcoverage metallic coatings that are placed on beads can be partiallyremoved by chemical etching.

The components of the beads are described as oxides, i.e., the form inwhich the components are presumed to exist in the completely processedglass and glass-ceramic beads as well as retroreflective articles, andthe form that correctly accounts for the chemical elements and theproportions thereof in the beads. The starting materials used to makethe beads may include some chemical compound other than an oxide, suchas a carbonate. Other starting materials become modified to the oxideform during melting of the ingredients. Thus, the compositions of thebeads of the present invention are discussed in terms of a theoreticaloxide basis. The compositions described herein are reported on atheoretical oxide basis based on the amounts of starting materials used.These values do not necessarily account for fugitive materials (e.g.,fugitive intermediates) that are volatilized during the melting andspheroidizing process.

The compositions of beads, discussed in terms of a theoretical oxidebasis, can be described by listing the components together with theirweight percent (wt-%) concentrations or their mole percent (mol-%)concentrations in the bead. Listing mol-% concentrations of componentsdemands care to be explicit about the chemical formulae to which the mol% figures are being applied. For example, in certain circumstances, itis convenient to describe lanthanum oxide by the chemical formula La₂O₃;however, in other circumstances it is more convenient to describelanthanum oxide by the chemical formula LaO_(3/2), The latter notationis an example of an approach where the chemical formula for a metaloxide comprising a single metal is adjusted to yield a single metal atomper formula unit and whatever quantity of oxygen atoms (even iffractional) is required to reflect accurately the overall stoichiometryof the metal oxide. For compositions expressed herein in terms ofconcentrations given in units of mol-% of metal oxides, the mol-%figures relate to such formula units that include a single, unitarymetal atom. Microspheres according to the present invention comprise atleast 40 wt-% titania (e.g., 41 wt-%, 42 wt-%, 43 wt-%, 44 wt-%),preferably at least 45 wt-% titania (e.g., 46 wt-%, 47 wt-%, 48 wt-%, 49wt-%), and more preferably at least 50 wt-% titania (e.g., 51 wt-%, 52wt-%, 53 wt-%, 54 wt-%, 55 wt-%, 56 wt-%, 57 wt-%, 58 wt-%, 59 wt-%).The amount of titania for the microspheres is typically less than 80wt-% (e.g., 79 wt-%, 78 wt-%, 77 wt-%, 76 wt-%, 75 wt-%, 74 wt-%, 73wt-%, 72 wt-%, 71 wt-%) and preferably no greater than 70 wt-% (e.g., 69wt-%, 68 wt-%, 67 wt-%, 66 wt-%). The amount of titania in at least someembodiments ranges from 60 wt-% to 65 wt-% (e.g., 61 wt-%, 62 wt-%, 63wt-%, 64 wt-%).

Titania is a high index of refraction metal oxide with a melting pointof 1840° C., and is typically used because of its optical and electricalproperties, but not generally for hardness or strength. Similar tozirconia, titania is a strong nucleating agent known to causecrystallization of glass materials. Despite its high individual meltingpoint, as a component in a mixture of certain oxides, titania can lowerthe liquidus temperature, while significantly raising the index ofrefration of microspheres comprising such mixtures of oxides.Compositions of the present invention comprising titania and optionallyzirconia provide relatively low liquidus temperatures, very high indexof refraction values, high crystallinity when heat-treatedappropriately, useful mechanical properties, and high transparency.

In some embodiments, microspheres described herein comprise at least 10wt-% (e.g., 11 wt-%, 12 wt-%, 13 wt-%, 14 wt-%) lanthanum oxide. Forsome embodiments, the amount of lanthanum oxide is at least 15 wt-%(e.g., 16 wt-%, 17 wt-%, 18 wt-%, 19 wt-%), at least 20 wt-% (e.g., 21wt-%, 22 wt-%, 23 wt-%, 24 wt-%) or at least 25 wt-% (e.g., 26 wt-%, 27wt-%, 28 wt-%, 29 wt-%, 30 wt-%, 31 wt-%, 32 wt-%, 33 wt-%, 34 wt-%).The amount of lanthanum oxide may range up to 60 wt-%. For someembodiments, the amount of lanthanum oxide ranges up to 55 wt-%. Theamount of lanthanum oxide of some preferred embodiments ranges from 25wt-% to 35 wt-%.

Lanthanum is one of a group of 15 chemically related elements in groupIIIB of the periodic table (lanthanide series). The names and atomicnumbers of the lanthanide series is as follows:

Element Symbol Atomic No. Lanthanum La 57 Cerium Ce 58 Praseodymium Pr59 Neodymium Nd 60 Promethium Pm 61 Samarium Sm 62 Europium Eu 63Gadolinium Gd 64 Terbium Tb 65 Dysprosium Dy 66 Holmium Ho 67 Erbium Er68 Thulium Tm 69 Ytterbium Yb 70 Lutetium Lu 71

Although promethium is a rare earth element, such element is believednot to be naturally occurring on earth. Due to the expense ofmanufacturing, promethium oxide is less preferred. Similarly lanthanumand gadolinium tend to be preferred due to their greater availability.Lanthanum oxide, gadolinium oxide, and combinations thereof, mayrepresent greater than 75 wt-% of the lanthanide series oxides of amaterial described herein. In some embodiments, lanthanum oxide,gadolinium oxide, and combinations thereof, represent at least 80 wt-%,at least 85 wt-%, at least 90 wt-%, at least 95 wt-%, and even 100% ofthe lanthanide series oxides.

In some embodiments, the microspheres may comprise oxides of otherlanthanide series elements in place of or in combination with lanthanumoxide. Accordingly, the microspheres of the invention may comprise oneor more oxides selected from oxides of the lanthanide series ofelements. Any of the previous ranges provided with respect to lanthanumoxide content can be adjusted based on the molecular weight of thechosen combination of lanthanide series oxides to provide the same molarratios. One preferred composition comprises 15 to 25 mol-% of one ormore oxides selected from the oxides of the lanthanide series ofelements; 4 to 8 mol-% zirconia, and 70 to 82 mol % titania.

The microspheres described herein optionally, yet typically comprise atleast 2 wt-% zirconia. The amount of zirconia ranges up to 40 wt-%. Theamount of zirconia is typically less than 30 wt-%. In one embodiment,the amount of zirconia ranges from about 5 wt-% to about 25 wt-%. Theamount of zirconia is preferably no greater than 10 wt-%. Generally, thezirconia contributes chemical and mechanical durability as well ascontributes to the high index of refraction of the preferred beads. Asis commonly known, zirconia often includes some level of hafnia (HfO₂)contamination. Also, it is known that hafnia as well as thoria (ThO₂)can exhibit similar physical and chemical properties to those ofzirconia. Accordingly, although beads of the present invention aredescribed in terms of their content of zirconia, it will be appreciatedby one of ordinary skill in the art that hafnia and thoria can besubstituted in part or in whole for zirconia.

In one embodiment, the micropheres comprise 45 wt-% to 70 wt-% TiO₂, 20wt-% to 55 wt-% La₂O₃, and 5 to 25 wt-% ZrO₂, HfO₂, ThO₂ and mixturesthereof.

In another embodiment, the microspheres comprise 60 wt-% to 65 wt-%TiO₂, 25 wt-% to 35 wt-% La₂O₃, and 5 to 10 wt-% ZrO₂, HfO₂, ThO₂, andmixtures thereof.

The microspheres may comprise at least 75 wt-%, 80 wt-%, 85 wt-%, andeven at least 90 wt-% TiO₂; lanthanide series oxides; and ZrO₂, HfO₂,ThO₂, and mixtures thereof.

In another embodiment, the pavement marking comprises transparentmicropheres comprising at least 5 mol-% Y₂O₃ optionally in combinationwith one or more lanthanide series oxides; at least 50 mol-% TiO₂ andoptionally at least 5 mol-% zirconia, hafnia, thoria, and mixturesthereof.

Microspheres described herein may comprise up to 25 wt-% (e.g., 1 wt-%,2 wt-%, 3 wt-%, 4 wt-%, 5 wt-%, 6 wt-%, 7 wt-%, 8 wt-%, 9 wt-%, 10 wt-%,11 wt-%, 12 wt-%, 13 wt-%, 14 wt-%) of other metal oxides. Such othermetal oxides are selected as to not detract from the higher refractiveindex properties of the microsphers. Other metal oxides may be selectedfor addition with the purpose of lowering the melting point of thematerial, leading to easier processing. Suitable other metal oxidesinclude for example LiO₂, Na₂O, K₂O, alkaline earth oxides such as BaO,SrO, MgO, and CaO, Al₂O₃, ZnO, SiO₂, and B₂O₃. Other metal oxides may beselected for addition with the purpose of improving the mechanicalproperties of the material. Typically, however, the amount of such othermetal oxides is typically less than 15 wt-%, less than 10 wt-%, or lessthan 5 wt-%. In some preferred embodiments, the composition issubstantially free (less than 1 wt-%) of any other metal oxides.

The glass-ceramic microspheres of the invention comprise one or morecrystalline phases, typically totaling at least 5 volume %.Crystallinity is typically developed through heat-treatment of amorphousbeads, although some glass-ceramic beads according to the invention andformed by quenching molten droplets may contain crystals withoutsecondary heat treatment. Such a crystalline phase or phases may includerelatively pure single-component metal oxide phases of titania (e.g.,anatase, rutile) and/or zirconia (e.g., baddeleyite). Also, such acrystalline phase or phases may include relatively pure multicomponentmetal oxide phases (e.g., ZrTiO₄). Such a crystalline phase or phasesmay include crystalline solid solutions that are isostructural withrelatively pure single-component or multicomponent metal oxide phases.Finally, such crystalline phase or phases may include at least oneheretofore unreported crystalline phase, in terms of crystal structureand/or composition. The compositions exhibit controlled crystallizationcharacteristics such that they remain transparent following heattreatments.

Colorants can also be included in the beads of the present invention.Such colorants include, for example, CeO₂, Fe₂O₃, CoO, Cr₂O₃, NiO, CuO,MnO₂, V₂O₅ and the like. Typically, the beads of the present inventioninclude no more than about 5% by weight (e.g., 1%, 2%, 3%, 4%) colorant,based on the total weight of the beads (theoretical oxide basis). Also,rare earth elements, such as praseodymium, neodymium, europium, erbium,thulium, ytterbium may optionally be included for color or fluorescence.Preferably, the microspheres are substantially free of lead oxide (PbO)and cadmium oxide (CdO).

The microspheres described herein can be prepared from a melt process.Microspheres prepared from a melt process are described herein as“fused.” For ease in manufacturing, it is preferred that the microspherecomposition exhibits a relatively low liquidus temperature, such as lessthan about 1700° C., and preferably less than about 1600° C. Typicallythe liquidus temperature is less than about 1500° C. Generally,formulations including those at or near a eutectic composition(s) (e.g.,binary or ternary eutectic compositions) will have lowest melting pointsin the system and, therefore, will be particularly useful.

Upon initial formation from a melt, beads are formed that aresubstantially amorphous yet can contain some crystallinity. Thecompositions preferably form clear, transparent glass microspheres whenquenched. Upon further heat treatment, the beads can developcrystallinity in the form of a glass-ceramic structure, i.e.,microstructure in which crystals have grown from within an initiallyamorphous structure, and thus become glass-ceramic beads. Upon heattreatment of quenched beads, the beads can develop crystallinity in theform of a nanoscale glass-ceramic structure, i.e., microstructure inwhich crystals less than about 100 nanometers in dimension have grownfrom within an initially amorphous structure, and thus becomeglass-ceramic beads. A nanoscale glass-ceramic microstructure is amicrocrystalline glass-ceramic structure comprising nanoscale crystals.It is also within the scope of the present invention to provide atransparent microbead that is mostly crystalline (i.e., greater than 50vol-% crystalline) directly after quenching, thus bypassing aheat-treatment step. It is believed that in such cases, employed coolingrates are not high enough to preserve an amorphous structure, but arehigh enough to form nanocrystalline microstructure.

For the purposes of the present invention, microspheres exhibiting X-raydiffraction consistent with the presence of a crystalline phase areconsidered glass-ceramic microspheres. An approximate guideline in thefield is that materials comprising less than about 1 volume % crystalsmay not exhibit detectable crystallinity in typical powder X-raydiffraction measurements. Such materials are often considered “X-rayamorphous” or glass materials, rather than ceramic or glass-ceramicmaterials. Microspheres comprising crystals that are detectable by X-raydiffraction measurements, typically necessary to be present in an amountgreater than or equal to 1 volume % for detectability, are consideredglass-ceramic microspheres for the purposes of the present invention.X-ray diffraction data can be collected using a Philips AutomatedVertical Diffractometer with Type 150 100 00 Wide Range Goniometer,sealed copper target X-ray source, proportional detector, variablereceiving slits, 0.2° entrance slit, and graphite diffracted beammonochromator (Philips Electronics Instruments Company, Mahwah, N.J.),with measurement settings of 45 kV source voltage, 35 mA source current,0.04° step size, and 4 second dwell time. Likewise as used herein “glassmicrospheres” refers to microspheres having less than 1 volume % ofcrystals. Preferably, the glass-ceramic microspheres comprise greaterthan 10 volume % crystals. More preferably, the glass-ceramicmicrospheres comprise greater than 25 volume % crystals. Mostpreferably, the glass-ceramic microspheres comprise greater than 50volume % crystals.

In preferred embodiments, the microspheres form a microcrystallineglass-ceramic structure via heat treatment yet remain transparent. Forgood transparency, it is preferable that the microspheres compriselittle or no volume fraction of crystals greater than about 100nanometers in dimension. Preferably, the microspheres comprise less than20 volume % of crystals greater than about 100 nanometers in dimension,more preferably less than 10 volume %, and most preferably less thanabout 5 volume %. Preferably, the size of the crystals in thecrystalline phase is less than about 20 nanometers (0.02 micrometers) intheir largest linear dimension. Crystals of this size typically do notscatter visible light effectively, and therefore do not decrease thetransparency significantly.

Beads of the invention can be made and used in various sizes. It isuncommon to deliberately form beads smaller than 10 micrometers indiameter, though a fraction of beads down to 2 micrometers or 3micrometers in diameter is sometimes formed as a by-product ofmanufacturing larger beads. Accordingly, the beads are preferably atleast 20 micrometers, (e.g., at least 50 micrometers, at least 100micrometers, at least 150 micrometers.) Generally, the uses for highindex of refraction beads call for them to be less than about 2millimeters in diameter, and most often less than about 1 millimeter indiameter (e.g., less than 750 micrometers, less than 500 micrometers,less than 300 micrometers).

Glass microspheres according to the invention can be prepared by fusionprocesses as disclosed, for example, in U.S. Pat. No. 3,493,403 (Tung etal). In one useful process, the starting materials are measured out inparticulate form, each starting material being preferably about 0.01micrometer to about 50 micrometer in size, and intimately mixedtogether. The starting raw materials include compounds that form oxidesupon melting or heat treatment. These can include oxides, (e.g.,titania, zirconia, and alkaline earth metal oxide(s)), hydroxides, acidchlorides, chlorides, nitrates, carboxylates, sulfates, alkoxides, andthe like, and the various combinations thereof. Moreover, multicomponentmetal oxides such as lanthanum titanate (La₂TiO₅) and barium titanate(BaTiO₃) can also be used.

Glass microspheres according to the invention can, alternatively, beprepared by other conventional processes as, for example, disclosed inU.S. Pat. No. 2,924,533 (McMullen et al) and in U.S. Pat. No. 3,499,745.The oxide mixture can be melted in a gas-fired or electrical furnaceuntil all the starting materials are in liquid form. The liquid batchcan be poured into a jet of high-velocity air. Beads of the desired sizeare formed directly in the resulting stream. The velocity of the air isadjusted in this method to cause a proportion of the beads formed tohave the desired dimensions. Typically, such compositions have asufficiently low viscosity and high surface tension. Typical sizes ofbeads prepared by this method range from several tenths of a millimeterto 3-4 millimeters.

Melting of the starting materials is typically achieved by heating at atemperature within a range of about 1500° C. to about 1900° C., andoften at a temperature, for example, of about 1700° C. A direct heatingmethod using a hydrogen-oxygen burner or acetylene-oxygen burner, or anoven heating method using an arc image oven, solar oven, graphite ovenor zirconia oven, can be used to melt the starting materials.

Alternatively, the melted starting material is quenched in water, dried,and crushed to form particles of a size desired for the final beads. Thecrushed particles can be screened to assure that they are in the properrange of sizes. The crushed particles can then be passed through a flamehaving a temperature sufficient to remelt and spheroidize the particles.

In a preferred method, the starting materials are first formed intolarger feed particles. The feed particles are fed directly into aburner, such as a hydrogen-oxygen burner or an acetylene-oxygen burneror a methane-air burner, and then quenched in water (e.g., in the formof a water curtain or water bath). Feed particles may be formed bymelting and grinding, agglomerating, or sintering the startingmaterials. Agglomerated particles of up to about 2000 micrometers insize (the length of the largest dimension) can be used, althoughparticles of up to about 500 micrometers in size are preferred. Theagglomerated particles can be made by a variety of well known methods,such as by mixing with water, spray drying, pelletizing, and the like.The starting material, particularly if in the form of agglomerates, canbe classified for better control of the particle size of the resultantbeads. Whether agglomerated or not, the starting material may be fedinto the burner with the burner flame in a horizontal orientation.Typically, the feed particles are fed into the flame at its base. Thishorizontal orientation is desired because it can produce very highyields (e.g., 100%) of spherical particles of the desired level oftransparency.

The procedure for cooling the molten droplets can involve air cooling orrapid cooling. Rapid cooling is achieved by, for example, dropping themolten droplets of starting material into a cooling medium such as wateror cooling oil. In addition, a method can be used in which the moltendroplets are sprayed into a gas such as air or argon. The resultantquenched fused beads are typically sufficiently transparent for use aslens elements in retroreflective articles. For certain embodiments, theyare also sufficiently hard, strong, and tough for direct use inretroreflective articles. A subsequent heat-treating step can improvetheir mechanical properties. Also, heat treatment and crystallizationlead to increases in index of refraction.

In a preferred embodiment, a bead precursor can be formed andsubsequently heated. As used herein, a “bead precursor” refers to thematerial formed into the shape of a bead by melting and cooling a beadstarting composition. This bead precursor is also referred to herein asa quenched fused bead, and may be suitable for use without furtherprocessing if the mechanical properties, index of refraction, andtransparency are of desirable levels. The bead precursor is formed bymelting a starting composition containing prescribed amounts of rawmaterials (e.g., titanium raw material, optional raw materials), formingmolten droplets of a predetermined particle size, and cooling thosemolten droplets. The starting composition is prepared so that theresulting bead precursor contains the desired metal oxides inpredetermined proportions. The particle size of the molten droplets isnormally within the range of about 10 micrometers to about 2,000micrometers. The particle size of the bead precursors as well as theparticle size of the final transparent fused beads can be controlledwith the particle size of the molten droplets.

In certain preferred embodiments, a bead precursor (i.e., quenched fusedbead) is subsequently heated. Preferably, this heating step is carriedout at a temperature below the melting point of the bead precursor.Typically, this temperature is at least about 750° C. Preferably, it isabout 850° C. to about 1000° C., provided it does not exceed the meltingpoint of the bead precursor. If the heating temperature of the beadprecursor is too low, the effect of increasing the index of refractionor the mechanical properties of the resulting beads will beinsufficient. Conversely, if the heating temperature is too high, beadtransparency can be diminished due to light scattering from largecrystals. Although there are no particular limitations on the time ofthis heating step to increase index of refraction, developcrystallinity, and/or improve mechanical properties, heating for atleast about 1 minute is normally sufficient, and heating shouldpreferably be performed for about 5 minutes to about 100 minutes. Inaddition, preheating (e.g., for about 1 hour) at a temperature withinthe range of about 600° C. to about 800° C. before heat treatment may beadvantageous because it can further increase the transparency andmechanical properties of the beads. Typically, and preferably,heat-treatment step is conducted in air or oxygen. These atmospheres aregenerally beneficial in improving color characteristic of beads, makingthem whiter. It is also within the scope of the present invention toconduct heat-treatment in an atmosphere other than air or oxygen.

The latter method of preheating is also suitable for growing finecrystal phases in a uniformly dispersed state within an amorphous phase.A crystal phase containing oxides of zirconium, titanium, etc., can alsoform in compositions containing high levels of zirconia or titania uponforming the beads from the melt (i.e., without subsequent heating).Significantly, the crystal phases are more readily formed (eitherdirectly from the melt or upon subsequent heat treatment) by includinghigh combined concentrations of titania and zirconia (e.g., combinedconcentration greater than 70%).

Microspheres made from a melt process are characterized as “fused.”Fully vitreous fused microspheres comprise a dense, solid, atomisticallyhomogeneous glass network from which nanocrystals can nucleate and growduring subsequent heat treatment.

The crush strength values of the beads of the invention can bedetermined according to the test procedure described in U.S. Pat. No.4,772,511 (Wood). Using this procedure, the beads demonstrate a crushstrength of preferably at least about 350 MPa, more preferably at leastabout 700 MPa.

The durability of the beads of the invention can be demonstrated byexposing them to a compressed air driven stream of sand according to thetest procedure described in U.S. Pat. No. 4,758,469 (Lange). Using thisprocedure, the beads are resistant to fracture, chipping, and abrasion,as evidenced by retention of about 30% to about 60% of their originalretroreflected brightness.

EXAMPLES

The following provides an explanation of the present invention withreference to its examples and comparative examples. Furthermore, itshould be understood that the present invention is no way limited tothese examples. All percentages are in weight percents, based on thetotal weight of the compositions, unless otherwise specified.

Test Methods

1. Wet patch brightness values were determined using a retroluminometer.The device directs white light onto a planar monolayer of microspheresdisposed on a white backing material at a fixed entrance angle to thenormal to the monolayer. Retroreflective brightness, patch brightness,is measured by a photodetector at a fixed divergence angle to theentrance angle (observation angle) in units of (Cd/m²)/lux. Datareported herein were measured at −4° entrance angle and 0.2° observationangle. Retroreflective brightness measurements were made for the purposeof comparison of brightness between beads of different composition. Thevalues were normalized by dividing by a constant factor greater than thehighest measured value. All measurements were made on samples with alayer of water with thickness about 1 millimeter on top of and incontact with the beads.2. X-ray diffraction was used to determine the crystallinity for certainexample microspheres. X-ray diffraction data can be collected using aPhilips Automated Vertical Diffractometer with Type 150 100 00 WideRange Goniometer, sealed copper target X-ray source, proportionaldetector, variable receiving slits, 0.2° entrance slit, and graphitediffracted beam monochromator (Philips Electronics Instruments Company,Mahwah, N.J.), with measurement settings of 45 kV source voltage, 35 mAsource current, 0.04° step size, and 4 second dwell time.3. Index of refraction of the microspheres was measured according to T.Yamaguchi, “Refractive Index Measurement of High Refractive IndexBeads,” Applied Optics Volume 14, Number 5, pages 1111-1115 (1975).

Examples 1-24 Starting Materials

The following starting materials were employed in the examples:zirconium oxide—commercially available from Z-TECH division of CarpenterEngineering Products, Bow, N.H., under the trade designation“CF-PLUS-HM”titanium oxide—commercially available from KRONOS Incorporated,Cranbury, N.J., under the trade designation “KRONOS 1000”barium carbonate—commercially available from Chemical ProductsCorporation, Cartersville, Ga., under the trade designation “Type S”lanthanum oxide—commercially available from Treibacher, Industrie Inc.,Toronto, Ontario, Canada, under the trade designation “Lanthanum OxideLa₂O₃, 99.9%”aluminum oxide—commercially available from ALCOA Industrial Chemicals,Pittsburgh, Pa., under the trade designation “16SG”, andgadolinium oxide—commercially available from Treibacher, Industrie Inc.,Toronto, Ontario, Canada, under the trade designation “Gadolinium OxideGd₂O₃, 99.99%”

Microsphere Preparation

For each example, the gram amounts of each metal oxide as specified inTable 1 as follows were combined in a 1 quart porcelain jar mill with 3g of sodium carboxymethylcellulose (commercially available from theAqualon Division of Hercules Incorporated, Hopewell, Va., under thetrade designation “CMC 7L2C”), approximately 350 g of water, andapproximately 1600 g of 1 cm diameter zirconium oxide milling media.

The resulting slurry was milled for approximately 24 hours and thendried overnight at 100° C. to yield a mixed powder cake with thecomponents homogeneously distributed. After grinding with a mortar andpestle, the dried and sized particles (<212 microns diameter) were fedinto the flame of a hydrogen/oxygen torch (commercially available fromBethlehem Apparatus Company, Hellertown, Pa. under the trade designation“Bethlehem Bench Burner PM2D Model-B”), referred to as “Bethlehemburner” hereinafter. The Bethlehem burner delivered hydrogen and oxygenat the following rates, standard liters per minute (SLPM):

Hydrogen Oxygen Inner ring 8.0 3.0 Outer ring 23.0 9.8 Total 31.0 12.8

The particles were melted by the flame and transported to a waterquenching vessel, yielding fused microspheres. The quenched particleswere dried and then passed through the flame of the Bethlehem burner asecond time, where they were melted again and transported to the waterquenching vessel. A portion of the quenched microspheres washeat-treated by heating at 10° C./minute to 850° C., holding at 850° C.for 1 hour, and furnace cooling.

Table 2 describes the theoretical bead composition for each example,accounting for decomposition of any carbonate that was present in theraw material batches. Table 2 also reports index of refraction valuesfor quenched microspheres i) after flame-forming and ii) after furnaceheat-treatment. Finally, Table 2 also reports the relative wet patchbrightness values for heat-treated microspheres that were sieved todiameter less than 106 micrometers. Values of relative wet patchbrightness for the sieved microspheres were approximately proportionalthe observed fraction of transparent microspheres present for each thesamples, that ranged from approximately 1 percent to approximately 90percent (i.e., greater fraction of transparent microspheres led tohigher relative wet patch brightness values).

TABLE 1 Example Gd₂O₃ La₂O₃ ZrO₂ TiO₂ Al₂O₃ BaCO₃ No. (g) (g) (g) (g)(g) (g) 1 35.2 26.6 138.2 2 51.8 13.0 135.2 3 67.6 132.4 4 34.4 39.0126.6 5 50.6 25.4 124.0 6 66.0 12.4 121.6 7 81.0 119.0 8 49.4 37.4 113.29 64.6 24.4 111.0 10 79.2 12.0 108.8 11 93.2 106.8 12 63.2 35.8 101.0 1377.6 23.4 99.0 14 91.4 11.6 97.0 15 33.6 50.8 115.6 16 48.4 48.8 102.817 25.4 25.6 125.0 30.8 18 33.4 12.6 122.6 40.4 19 25.0 37.6 114.0 30.220 49.6 24.8 121.6 4.0 21 64.8 12.2 119.0 4.0 22 57.2 18.6 120.2 4.0 2355.0 16.6 124.4 4.0 24 50.0 30.0 120.0

TABLE 2 Index of Wet Patch Refraction Index of Brightness afterRefraction after Heat- Example Flame- after Heat- Treatment No. Gd₂O₃La₂O₃ ZrO₂ TiO₂ Al₂O₃ BaO Forming Treatment (relative) 1 17.6 13.3 69.1N/A N/A 0.03 2 25.9 6.5 67.6 2.39 2.48 0.23 3 33.8 66.2 2.37 2.44 0.69 417.2 19.5 63.3 N/A N/A 0.03 5 25.3 12.7 62.0 2.40 2.44 0.94 6 33.0 6.260.8 2.39 2.43 0.95 7 40.5 59.5 2.35 2.34 0.18 8 24.7 18.7 56.6 2.362.41 0.93 9 32.3 12.2 55.5 2.34 2.35 0.51 10 39.6 6.0 54.4 2.34 2.340.80 11 46.6 53.4 2.31 2.32 0.61 12 31.6 17.9 50.5 2.33 2.33 0.78 1338.8 11.7 49.5 2.31 2.32 0.73 14 45.7 5.8 48.5 2.28 2.31 0.66 15 16.825.4 57.8 N/A 2.35 0.10 16 24.2 24.4 51.4 2.33 2.37 0.65 17 12.7 12.862.5 12.0 2.33 2.37 0.87 18 16.7 6.3 61.3 15.7 2.33 2.39 0.78 19 12.518.8 57.0 11.7 2.31 2.38 0.10 20 24.8 12.4 60.8 2.0 2.34 2.42 0.75 2132.4 6.1 59.5 2.0 2.31 2.39 0.82 22 28.6 9.3 60.1 2.0 2.34 2.44 0.78 2327.5 8.3 62.2 2.0 2.33 2.43 0.88 24 25.0 15.0 60.0 2.32 2.42 0.11

FIG. 4 gives a plot of measured X-ray diffraction data for microspheresof Example 18, sieved to less than 106 micrometers in diameter. Beforeheat-treatment, the microspheres are judged to be largely X-rayamorphous, with any crystallinity present at a level of less than 10% byvolume. After heat-treatment, the transparent microspheres are judged toinclude approximately 40% crystallinity by volume, as evidenced by theappearance of crystalline features in the X-ray diffraction plot.

Example 25 Pavement Marking

A flat piece of asphalt-bonded aggregate paving cement measuringapproximately 4 centimeters in thickness, approximately 8 centimeters byapproximately 15 centimeters in area, and approximately 850 grams inmass was placed, together with an approximately 7.6 gram piece of yellowalkyd thermoplastic pavement marking binder resting on it (commerciallyavailable from Ennis Paint, Inc (Atlanta, Ga.) under the tradedesignation Pave-Mark Y5E-5), in a convection oven at 200° C. Afterheating at 200° C. for 45 minutes, the binder had softened and flowed toa relatively flat coating layer on the cement. Heat-treated microspheresof Example 17, sieved to less than 106 micrometers in diameter, weresprinkled on top of the hot binder-coated cement. Under an opticalmicroscope, the microspheres were observed to have embedded into thebinder to about half of their diameters. After the marked paving cementhad cooled, it was observed from a distance of several meters with aflashlight for retroreflectivity. When dry, it displayed limitedretroreflectively. When submerged in water, it displayed substantiallygreater, and easily observable, retroreflectivity.

Example 26 Pavement Marking

A flat piece of portland cement-type paving material measuringapproximately 2 centimeters in thickness, approximately 8 centimeters byapproximately 9 centimeters in area, and approximately 270 grams in masswas placed, together with an approximately 15.6 gram piece of the yellowalkyd thermoplastic pavement marking binder resting on it, in aconvection oven at 200° C. After heating at 200° C. for 25 minutes, thebinder had softened and flowed to a relatively flat coating layer on thecement. Heat-treated microspheres of example 17, sieved to less than 106micrometers in diameter, were sprinkled on top of the hot binder-coatedcement. Under an optical microscope, the microspheres were observed tohave embedded into the binder to about half of their diameters. FIG. 5is an optical photomicrograph of the partially embedded beads. After themarked paving cement had cooled, it was observed from a distance ofseveral meters with a flashlight for retroreflectivity. When dry, itdisplayed limited retroreflectively. When submerged in water, itdisplayed substantially greater, and easily observable,retroreflectivity.

1. A retroreflective article, comprising: first transparent microsphereshaving an index of refraction of at least 2.10 and comprising: at least50 mol-% TiO₂; and at least 5 mol-% of one or more metal oxides selectedfrom oxides of lanthanide series elements and yttrium; and secondtransparent microspheres having an index of refraction within a rangefrom 1.5 to 2.0; wherein the first and second transparent microspheresare at least partially embedded in a binder.
 2. The retroreflectivearticle of claim 1, wherein the first microspheres have an index ofrefraction of at least 2.30.
 3. The retroreflective article of claim 1,wherein the first microspheres include a glass-ceramic structure.
 4. Theretroreflective article of claim 1, wherein the first microspherescomprise at least 25 wt-% of one or more metal oxides selected fromoxides of the lanthanide series elements and yttrium.
 5. Theretroreflective article of claim 1, wherein the first microspherescomprise between about 5 wt-% and about 15 wt-% of at least one metaloxide selected from ZrO₂, HfO₂, ThO₂, and mixtures thereof.
 6. Theretroreflective article of claim 1, wherein the first micropherescomprise: 45 wt-% to 70 wt-% TiO₂; 20 wt-% to 55 wt-% of one or moremetal oxides selected from oxides of the lanthanide series elements andyttrium; 5 to 25 wt-% of at least one metal oxide selected from ZrO₂,HfO₂, ThO₂, and mixtures thereof.
 7. The retroreflective article ofclaim 1, wherein the first microspheres include at least 90 wt-% of atleast one metal oxide selected from TiO₂, La₂O₃, ZrO₂, HfO₂, ThO₂ andmixtures thereof.
 8. The retroreflective article of claim 1, wherein thefirst microspheres comprise: 60 wt-% to 65 wt-% TiO₂; 25 wt-% to 35 wt-%of one or more metal oxides selected from oxides of the lanthanideseries elements and yttrium; and 5 to 15 wt-% of at least one metaloxide selected from ZrO₂, HfO₂, ThO₂, and mixtures thereof.
 9. Theretroreflective article of claim 1, wherein the binder includes apigment selected from at least one diffusely reflecting pigment, atleast one specularly reflecting pigment, and combinations thereof. 10.The retroreflective article of claim 1, wherein the microspheres arefused.
 11. The retroreflective article of claim 1, wherein themicrospheres include a reflective coating.
 12. The retroreflectivearticle of claim 1, wherein a transparent binder is between themicrospheres and a reflective layer.
 13. The retroreflective article ofclaim 1, wherein the microspheres comprise at least 10 wt-% of at leastone metal oxide selected from La₂O₃, Gd₂O₃, and mixtures thereof.
 14. Aretroreflective element, comprising: a core; and transparentmicrospheres at least partially embedded in the core and having an indexof refraction of at least 2.10; wherein at least a portion of thetransparent microspheres include at least 40 wt-% TiO₂ and at least 10wt-% of one or more lanthanide series oxides.
 15. The retroreflectiveelement of claim 14, wherein the core includes at least one of anorganic material, an inorganic material, or a mixture thereof.